1. Field of the Invention
The present invention relates to a superconducting device, and more specifically to a superconducting device composed of a normal conductor layer sandwiched between a pair of superconductor electrodes so as to separate the pair of superconductor electrodes from each other in close proximity to each other, so that superconduction current flows through the normal conductor layer.
2. Description of Related Art
One typical example of superconductor technology applied to electronic devices is a Josephson junction formed of a weak linkage between a pair of superconductor electrodes. A Josephson junction may be comprised of various type junctions, among which a junction called a "tunnel type" is said to have the most preferable characteristics. The tunnel type Josephson junction device is currently used in various superconductive elements such as high sensitive sensors, superconductive transistors, etc. The tunnel Josephson junction is basically comprised of a pair of superconductor electrodes separated by a non-superconductor layer sandwiched between the pair of superconductor electrodes so that the pair of superconductor electrodes are in close proximity to each other but not in direct contact with each other.
In superconducting devices, including the above mentioned tunnel Josephson junction, each of the superconductor electrodes can be formed of various superconductive material. Examples of superconductive material from which the superconductor electrodes can be formed are metal type superconductive materials and so-called compound oxide superconductive materials. The non-superconductor layer sandwiched between the pair of superconductor electrodes can be formed of a material selected from a group consisting of insulator materials and normal conductor materials. The selection of material from which the non-superconductor layer is formed is based on the use of the superconductive device.
Furthermore, in the superconducting devices, including the tunnel Josephson junction, the thickness of the non-superconductor layer, defined as the distance between the pair of superconductor electrodes separated by the non-superconductor layer, is very significant. Determination of the preferred value of the distance between the pair of superconductor electrodes is a function of the coherence length of superconductive electrons in the non-superconductor layer and of the coherence length of the superconductor electrodes. If this distance is too large or to small, proper operation of the Josephson junction cannot be insured.
In general, compound oxide superconductive materials have a short coherence length. Therefore, for a superconducting device in which the superconductor electrodes are comprised of a compound oxide superconductive material it is difficult to select both a functional non-superconductive material and a functional thickness of the non-superconductor material layer.